If the Common port BMS sees a condition that disconnects the battery from its output won't that cause the SCC to be disconnected from the battery and allow the solar panels to damage the SCC? If you make a secondary connection directly to the battery bypassing the BMS connection that will assure a power source to the SCC so as not to damage the SCC from a solar panel connection without power to the battery connection of the SCC, but won't that also allow the solar panels via the SCC to supply a charging current to the battery even if the low temp cutoff of the BMS activates? Seems to me you are either risking damage to the SCC if the BMS disconnects power or you are risking damage to the LiFePO4 battery if the temperature drops below 0 C and you have a direct connection from the SCC with no protection from the BMS. Is this correct or am I missing something here?
BMS question
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John Frum
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Depends on the BMS some common port bms are physically common and logically separate.
I'm talking about voltage on the battery connection of a SCC from the battery/BMS so that the voltage from the solar panels won't damage the SCC. The charge controllers I have require a battery to be connected before you connect the solar panels or it will damage the SCC. If I'm wrong I'd love to have someone explain why. But it seems that a low voltage cutoff on a BMS would remove power from the SCC and the solar panels would damage it.
John Frum
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Will Prowse tested that and was not able to damage a solar charge controller.
I believe he tried a few.
And as I said it depends on the BMS if both charge and discharge paths will be disabled or not.
What SCC do you have?
The Epever Tracer 3215BN, it's a 30a charge current, 150 volt PV max input voltage. That's why I'm so confused about the possible damage from the PV input, if it's rated for 150 volts my three panels in series won't give more than 65 volts. I'd like to see the schematic of the unit, maybe that would give a clue as to why input voltage at the PV terminals without power supplied to the battery input could cause a problem.
One reason I am concerned about it is that Will does go out of his way to emphasize disconnecting the PV side before you remove the battery when working on the system. I suspect he may have smoked a few units in his time. I like to learn from others mistakes but it's nice to also understand why it was a mistake.
Do you recall which video that's on? I'd like to see what he was using.
John Frum
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Not sure if it was a video but there was a forum post.
I recall the forum post but am unable to find it using the search feature.
"Trying to Destroy my SCCs: I Can't Do it!"
Here is one excerpt from the post pertaining to Victron that may be relevant:
Here is one excerpt from the post pertaining to Victron that may be relevant:
This has been an ongoing debate for some time. Intuitively you are correct, and most SCC manuals explicitly state that you must always have a battery connected, but learned experience (from members of this forum and tests carried out by Will, as linked above) seem to indicate that this is not an issue.
Who do you trust? SCC manufacturer or learned experience? That's why it's an ongoing debate. Search the forum - lots of posts about this.
Personally, I've disconnected my batteries loads of times, never been an issue, but then I do have a rather expensive SCC.
Who do you trust? SCC manufacturer or learned experience? That's why it's an ongoing debate. Search the forum - lots of posts about this.
Personally, I've disconnected my batteries loads of times, never been an issue, but then I do have a rather expensive SCC.
Adding to this what does the manufacturer of your SCC actually say. Do they explicitly caution you not to do this in the manual?
If Victron is representative of the industry (not sure that it is--especially with cheaper SCC's) manufacturers are moving away from prohibiting or warning against this.
Interestingly, I remember the caution in the manual when I first read it but I've just downloaded the latest version and that caution is nowhere to be seen. It does still mention that you must connect the battery first, but nothing about subsequent battery disconnections any more. Or I could have just dreamt the whole thing - wouldn't surprise me.
I can't remember what brand SCC you have?
gnubie
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Pretty confident I read that too. I'll see if I can find the manual for my bluesolar, its a few years old now. The ministry can't re-write my paper!
According to Justin, they did caution against it in the past but have been slowly editing it out of their manuals as they update them, as its no longer thought to be an issue:
Sounds definitive but don't forget Victron products are usually pretty well built - this isn't a definitive answer for all SCCs out there. But it is at the very least consistent with the learned experiences of those on here.
Agreed, I only mean for this to apply to Victron stuff. I don' t know about other controllers (I believe Will tested the Epever controllers, but not sure what the official word in their manuals is). Everyone should definitely follow the reccomendations of their manufacturer.
This is the sorta thing, that I would only worry about if the manufacturer explicitly warns about it. If the manual gives no indication one way or another, I would be fairly confident that (1) its not going to cause problems (2) it would be covered under warranty if it did. There are some things I'm more conservative about, but on this issue , I would want an explicit warning in the documentation.
I think you are right on. I contacted the manufacturer of the Epever Tracer3215BN and they said do not connect PV without battery connected first, but they wouldn't give any technical explanation. I think it comes down to the load and maybe the total open circuit voltage of the PV panels at the time.
Pierre
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I think it is to pre-load the output of the SCC as well as to supply a reference voltage of the battery as a lot of SCC ‘s are auto select multi voltage units these days. But I was also told before that this practice is only required when first switching the system on and thereafter the SCC ‘remembers’ the voltage of the battery it was connected to. If the battery was uprated say to 24v from 12v , the SCC will always revert to 12v first and then self adjust to 24v in a few seconds.
gnubie
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The Victron controllers do remember the battery voltage at the first start up from new or a reset-to-default. If it can't sense battery at the first startup it assumes 12v. There was a change to behaviour several firmware versions back but I can't remember what the change was.
Some controllers go a bit nuts without battery present. The MakeSkyBlue is one of them. It won't turn on without about 10v on the battery input so that mitigates it a bit but if battery goes away once started better hope you don't have any voltage sensitive loads connected as they won't have a good day.
Some controllers go a bit nuts without battery present. The MakeSkyBlue is one of them. It won't turn on without about 10v on the battery input so that mitigates it a bit but if battery goes away once started better hope you don't have any voltage sensitive loads connected as they won't have a good day.
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